The present invention relates, in general, to fossil fuel boilers and, in particular, to a new and useful method and arrangement for optimizing scheduled timing of sootblowing in such boilers.
The combustion of fossil fuels for the production of steam or power, generates a residue broadly known as ash. All but a few fuels have solid residues, and in some instances, the quantity is considerable.
For continuous operation, removal of ash is essential. In suspension firing the ash particles are carried out of the boiler furnace by the gas stream and form deposits on tubes in the gas passes (fouling). Under some circumstances, the deposits may lead to corrosion of these surfaces.
Some means must be provided to remove the ash from the boiler surfaces since ash in its various forms may seriously interfere with operation or even cause shutdown. Furnace wall and convection-pass surfaces can be cleaned of ash and slag while in operation by the use of sootblowers using steam or air as a blowing medium. The sootblowing equipment directs product steam through retractable nozzles aimed at the areas where deposits accumulate.
The convection-pass surfaces in the boiler, sometimes referred to as heat traps, are divided into distinct sections in the boiler, e.g. superheater, reheater, and economizer sections. Each heat trap normally has its own dedicated set of sootblowing equipment. Usually, only one set of sootblowers is operated at any time, since the sootblowing operation consumes product steam and at the same time reduces the heat transfer rate of the heat trap being cleaned.
Scheduling and sequencing of sootblowing is usually implemented with timers. The timing schedule is developed during initial operation and startup of the boiler. In addition to timers, critical operating parameters, such as gas side differential pressure, will interrupt the timing schedule when emergency plugging or fouling conditions are detected.
The sequencing, scheduling, and optimizing of the sootblowing operation can be automated by using controls. See U.S. Pat. No. 405,840 now abandoned filed Aug. 6, 1982 and entitled SOOTBLOWING OPTIMIZATION, which is here incorporated by reference.
The scheduling is usually set by boiler cleaning experts who observe boiler operating conditions and review fuel analyses and previous laboratory tests of fuel fouling. The sootblower schedule control settings may be accurate for the given operating conditions which were observed, but the combustion process is highly variable. There are constant and seasonal changes in load demand and gradual long term changes in burner efficiency and heat exchange surface cleanliness after sootblowing. Fuel properties can also vary for fuels such as bark, refuse, blast furnace gas, residue oils, waste sludge, or blends of coils. As a result, sootblowing scheduling based on several days of operating cycles may not result in the most economical or effective operation of the boiler.
Present practice for sootblowing scheduling is based on the use of timers. The timing schedule is developed during initial operation and startup, and according to the above application, can be economically optimized for constant and seasonal changes in load demand, fuel variations, and gradual long term changes in burner efficiency and heat exchange surface cleanliness after sootblowing.
A boiler diagnostic package which can be used for sootblowing optimization has been proposed by T. C. Heil et al in an article entitled "Boiler Heat Transfer Model For Operator Diagnostic Information" given at the ASME/IEEE Power Gen. Conference in October 1981 at St. Louis, Mo. The method depends upon estimates of gas side temperatures from coupled energy balances, and the implementation requires extensive recursive computations to solve a series of heat trap equations.
As noted, various approaches have been developed to optimize the use of sootblowing equipment. A method by Klatt and Matsko computes optimum sootblowing schedules using a model of boiler fouling characteristics which is adapted on-line. An identification of the rate of change of total boiler efficiency versus time ("fouling rate") is computed for multiple groupings of sootblowers in the various heat traps using only a measure of relative boiler efficiency. Using this information, the economic optimum cycle times for sootblower operation are predicted.
For the above scheme and others similar to it, a critical part of the computation is the identification of the "fouling rates". A major problem in this identification is the interaction of the effects due to multiple heat trap operations. Klatt and Matsko have assumed these effects to be negligible in their scheme, while other methods require a large number of additional inputs attempting to account for these interactions. For some combustion units with sootblowers, neglecting multiple heat trap interactions is valid (i.e., utility boilers). However, for many units sootblowing is a continuous procedure and a method of accounting for the interactions is necessary. This method should be implemented without adding a large number of expensive inputs.